1. Field of the Invention
Embodiments of the invention relate to solid state memory, and more particularly, in one or more embodiments, to dynamic random access memory.
2. Description of the Related Art
Solid state memory devices have been widely used as data storage in various electronic devices. Generally, solid state memory devices include volatile memories (e.g., dynamic or static random access memories) and non-volatile memories (e.g., read-only memories and flash memories).
Recently, the data processing speed of electronic devices such as a personal computer has been significantly improved. Thus, there has been a need for memory devices with a high data transfer rate which is compatible with such electronic devices. For certain memory devices, double data rate (DDR) schemes have been widely used for obtaining a high data rate during read or write operation. Memory devices using a DDR scheme transfer data on both the rising and falling edges of an external clock provided by an associated electronic device, effectively nearly doubling the data transfer rate. Examples of DDR schemes include DDR, DDR2, and DDR3.
The DDR schemes are typically combined with multi-bit prefetch schemes. Under the multi-bit prefetch schemes, for each of the data pins of a memory device, a plurality of bits of data are retrieved in parallel from a memory array in response to a single read command. The plurality of bits are stored in latch devices (or prefetch buffers) arranged in parallel. Then, the bits are multiplexed and output via the data pin on the rising and falling edges of an external clock issued from the associated electronic device. The numbers of bits prefetched in parallel under the DDR1, DDR2, and DDR3 schemes are 2, 4, and 8, respectively. A memory device using the DDR1 scheme prefetches 2 bits of data in parallel from memory cells in response to a single read command, and then outputs the 2 bits through a single data pin for a single external clock cycle. A memory device using the DDR2 scheme prefetches 4 bits of data in parallel from memory cells in response to a single read command, and then outputs the 4 bits through a single data pin for two external clock cycles. A memory device using DDR3 scheme prefetches 8 bits of data in parallel from memory cells in response to a single read command, and then outputs the 8 bits through a single data pin for three external clock cycles. Typically, the DDR2 scheme uses a higher external clock frequency than the DDR1 scheme. The DDR3 scheme typically uses a higher external clock frequency than the DDR2 scheme.